I understand driving them but what is the best way to fill them up without a jerrycan i’ve been using gallon jugs so far is there a better early game alt to them and how do I lvl up mechanics without reading at 0 also what are some easy improvements to make to early game cars
There’re two possible ways to raise mechanics from 0:
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Check the crafting menu and search for items that you can craft which have “mechanics” as their primary skill
or
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seal (the (*)construction menu has that option) and pry crates repeatedly until you level up.
There’s also a third option which involves removing simple parts from vehicles. I don’t remember which parts they are, but if you see a vehicle outside, you can rem[o]ve items at level 0. Some you may be able to reinstall, others you will need level 1, but if gaining exp is what you want, just remove whatever you can from the vehicles you come across that you know you’ll never use.
As to filling up vehicles, that’s very circumstancial. Jerrycans are a safe way to do it if you are [s]iphoning from others, as you can carry a jerrycan with you very easily without too many penalties. However if you can find and deconstruct water tanks such as the ones you find on top of Evac Shelters or in house basements, you get 60L tanks, which you can certainly use to fill with fuel or clean water but you can only wield them or drag them around on the floor.
As for easy improvements for early game cars… once again it depends on what resources you have, your skills, your game conditions and what your plan for the future is.
From finding solar cars and removing their solar panels and a Large Electric Engine to install on your own vehicle when you can, to finding Cargo Carriers instead of “trunks” to increase the volume you can carry in your vehicle. Also Boom Crane / Telescopic crane.
The game doesn’t have different kinds of plastics, so you don’t run the risk of your gallon jug (or plastic bottle) getting dissolved by the gas. Plastic jerrycans are reasonably convenient when it comes to weight/volume, with smaller containers requiring more repetitions to empty one tank into another.
Mech training: Parts removal, parts installation, parts repair.
Early modifications: Repair parts you can repair, in particular seat belts and wheels (“repair” in this context may also mean replace a part in bad condition with one in a good condition taken from another vehicle). Note that the tools you have available may limit what you can do as well as your skill level.
Other reasonably early mechanics thing: Build small utility “vehicles” similar to shopping carts for hauling, in particular if you’ve got the parts and skills for folding ones you can bring with you, but even a non folding one with a 60L tank is useful for fetching water (salt or fresh)
Yet another way of filling the vehicle with a liquid is to drive next to it so you can stand on a tile adjacent to vehicle from where you can also activate the source of liquid. Then activate it and there will be “refill nearby vehicle X” menu option.
The game doesn’t have any chemical interaction system outside recipes, so you can even store nitric acid in your steel vehicle tank for years 
That’s statement isn’t quite accurate. There are - very limited - chemical interactions to be aware of.
First, acid on the ground burns biological beings, including your character.
Second, drawing acidic blood (or acid) from zombies or other acid bearing monsters can damage the blood drawing kit.
But yes, gasoline eating through plastic is not a thing… yet.
alright thanks this helps a ton